Help with diffuser removal

[Observer]

Pics below are of light fitting in a rented flat that my daughter is using at uni, several hundred miles from me. The lamp needs replacing but she can't remove the cover/diffuser.

It looks to me as though it just clips on and can be released by a tool/lever in the slot pictured, which she has tried but maybe not with enough force. Can anyone confirm or suggest a better technique?

TIA

 

[flameport]

50p coin in the slot and twist.
If it's been on there for years, significant effort will be required.

 

[crystal ball]

Maybe try the 50p gripped in pliers, a bit of gentle tapping round the rim might help

 

[RF Lighting]

We use a 2p where I come from

 

[crystal ball]

Thats Yorkshire for ya